Planar transformers and planar inductors are used in a wide variety of products, and are typically used when the space available within a given product or device does not allow for placement of a conventional wire wound transformer. In general, planar transformers have a lower profile than conventional transformers for similar electromagnetic performance, and can thus be used in low profile product enclosures or packages where height restrictions prohibit the use of conventional transformers. Planar transformers and planar inductor achieve the necessary performance in low profile assemblies by using spiral windings.
Spiral windings are comprised of a conductor disposed on a flat substrate, such as, for example, a printed circuit board. In many applications spiral windings are stacked on a circuit board, with each winding on a separate layer. In making a planar inductor, the spiral windings are electrically coupled in series such that current through the windings flows in the same direction through each spiral. Meaning that if current is flowing in a clockwise direction, for example, it flows in a clockwise direction through each spiral conductor in connected in series to make an inductor or transformer winding. When the current reverses direction, the current flows in a counter clockwise direction through each spiral conductor, so as have an additive effect on the magnetic field produced by the current through each spiral conductor. In making a planar transformer, selected windings are electrically coupled in series to form primary and secondary windings, each comprising at least one spiral conductor. Typically the winding layers of the primary and secondary windings are interleaved to optimize electromagnetic performance. Once the winding layers are configured as needed, a core is placed around the windings to contain the magnetic field of the windings. In conventional planar inductors and transformers, the core completely covers the windings to capture the most magnetic flux possible.
The fact that the core completely covers the spiral conductor windings presents a problem. In planar devices used for power applications, such as power supplies, heat generated by the current through the windings becomes significant, and degrades the performance of the core and winding(s), and thus degrades the performance of the transformer or inductor. Unlike conventional bobbin style transformers, because spiral windings in conventional planar transformers or planar inductors are covered by the core and other winding layers, cooling the planar electromagnetic device is a significant issue.
A conventional technique for cooling the planar device is the use of a fan. However, this obviously adds expense and complexity to the product in which the planar device is used. Furthermore, while a fan can significantly cool the outer portions of the planar device, the internal regions will likely remain at high temperatures. Therefore there exists a need for a means by which a planar transformer or planar inductor can be efficiently cooled without the use of a fan, and such that the internal regions of the planar device will benefit.